396 PART III. THE CLASSIFICATION OF PLANTS 



Adventitious bads, subserving vegetative propagation, are com- 

 monly produced ; they arise most frequently on the subterranean 

 portions of leaf-stalks (as in Pteris aquilina, Aspidium Filix-mas), 

 and sometimes, as in Onoclea Struthiopteris, the bud grows into 

 a subterranean stolon which eventually throws up at its apex a 

 whorl of green leaves, thus constituting a new plant; but also 

 frequently from the lamina, as in Asplenium (Diplazium) celtidi- 

 folium, A. bulbiferum, and other species. The bud originates from 

 a single epidermal cell. 



General Histology. The structure of stem, petiole, and root, is 

 characterised throughout by the presence of hypodermal layers, 

 and, generally, of scattered strands of sclerenchymatous tissue, 

 consisting of more or less elongated ground- tissue cells with more 

 or less thickened brown-coloured walls ; and by the predominance 

 of scalariform vascular tissue in the xylem which consists, with 

 but few exceptions, of tracheides. 



The stem is, at its first development, monostelic, with a single 

 axile stele: in some forms this structure obtains (with or with- 

 out pith) throughout the whole stem (e.g. Hymenophyllacese, 

 Lygodium, Schiznoa, stolons of Nephrolepis) : in the Osmundacese 

 the stem is monostelic throughout, the stele eventually consisting 

 of a ring of bundles enclosing a pith : in the other families the 

 stem becomes polys teltc. 



In the monostelic stem the bundles are sometimes conjoint and 

 collateral (e.g. Trichomanes among Hymenophyllaceae, Osmun- 

 daceo>) : in all other cases the arrangement of the bundles in the 

 stele is concentric, or, more strictly speaking, bicollateral, since 

 the phloem does not quite completely surround the xylem-bundles. 

 The concentric steles are cauline and diarch, with usually an 

 endodermis and a pericycle : in some cases, however, where the 

 stele is small (e.g. some species of Polypodium) there is no peri- 

 cycle, its place being taken by a layer of cells formed by the 

 division of the primitive endodermis (p. 165) into two layers. 



In the polystelic stem the course of the steles is such that they 

 form a meshwork, each mesh corresponding to the insertion of a 

 leaf : the bundles of the leaf join those forming the corresponding 

 mesh in the stem. The form of the mesh is determined by the 

 number and insertion of the leaves: when the leaves are numerous 

 and closely arranged, the meshes are relatively short and broad; 

 when the leaves are few and scattered, the meshes are long and 

 narrow. Tn dorsiventral stems, a regular meshwork is only 



